Magnetron



May 21,' 1940 l H. E. Ho-LLMANN 2,201,666

MAGNETRON Filed March 4. 1938 e MUA$- @dk .d E@ a 'Vm/www@ WAGn/T/C/FH@ eil @Zz Afa sf/ef/l/qr/e/f/ 2 MAGNEr/z/NG CU aC/W- srRA/GHTL/NE RESSTANCE (LOAD) UNE MAGNET/ZIN@ pgAsoL/x r 10/ fff Q1., ;,V/ A' i o Il I *f* l I /H fb f 0 f" 43m gil/g :s H5 6 7 q m f 12ga# f5 @a W C/ INVENTOR.

HNS ERICH HOLLMANN BY W@ ATTORNEY.

Patented May 21, 1940 Unire STATES MAGNETRON Hans Erich Hollmann, Berlin, Germany, assignor to Telefunken Gesellschaft fr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application March 4,

1938, Serial No. 193,816

In Germany May 9, 1936 3 Claims.

My invention relates to electron discharge devices utilizing the phenomena of secondary emission and magnetic control and sometimes referred to as an electron multiplier. The present application describes and claims an improvement over my invention described and claimed in my application Serial No. 133,316, filed March 2:7, 1937, Patent No. 2,171,212, dated August 29, 1939 and assigned to the same assignee as the present application. l

In the magnetron described in my zzo-pending application identified above the electrons which carry the electron current are produced by secondary emission of cold cathodes. When switching in the electrical fields, at rst only a few electrons, for instance photo-electrons, leave the cathode and after describing a circular course they impinge on the second cathode coating. By means of a special construction oi the oscillatory tube in conjunction with ultra-high frequency resonance systems care is taken that the electrons retain a suflicient residual velocity when impinging on the second cathode so that a large number of secondary electrons are rey, 'leased from the cathode coating. The secondary electrons produced in great numbers by the impinging first primary electrons, now leave the second cathode coating in the reverse direction, and return to the rst cathode coating after describing their own circular course. At this cathode coating secondary electrons are produced anew, and the entire performance repeats itself.

Through alternative building up of the secondary emission from the two cathode coatings the i x emission and the .plate current will be gradually built up. In fact, this building up is limited only by the yield of the coatings having secondary emission, since in View oi the impulse like emission, space charges which ll in only a small i part of the electrode space play only a minor role.

In view of the fact that in the interest of an intensive building up of oscillations, it is aimed at imparting to the cathode coatings a possibly high yield in order that they deliver in the form of secondary electrons an amount equal 10 to 20 times the number of primary electrons impinging on said coatings, danger exists that the emission will be driven in this manner to a higher intensity than the tube can withstand from the point of View of heat. Under these conditions any practical operation is of course out of the question, and measures must be resorted to which prevent the emission current from growing beyondthe allowable value.

(Cl. Z50-27) The object of the present invention is to substantially eliminate an uncontrollable building up of the emission, since at a high yield of the cathode coating capable of secondary emission, the plate current may increase beyond the allowable load limit thereby destroying the tube.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a graph showing operating charac- Iteristics of a tube made according to my copending application identied above, Figure 2 is a like graph showing further operating characteristics under other conditions, and Figure 3 is a perspective of a tube and a diagram of a circuit made according to my invention.

In the tube arrangements operating with secondary emission such as have hitherto been known, the danger of unduly high currents was avoided simply by the insertion of an ohmic protecting resistor in the plate circuit of the tube. Such series resistor although limiting the emission current to a certain degree, however, renders on the other hand the operating conditions of the tube inexact and unstable as can be seen from the diagram of Figure 1 showing the electrical conditions.

The diagram reveals the action of an ohmic resistor placed ahead of the secondary emission magnetron of the type described in my co-pending application. The abscissa indicates the plate potential es of the tube, and the ordinate shows the plate current ia. It is at irst assumed that certain operating conditions are adjusted to, i. e. the magnetic eld is assumed to have the value Hr and the tuning of the generator circuit is maintained on the wavelength determined by the equation kozl/Hk. Furthermore, a limited yield of the cathode coatings having secondary emission is presupposed. A building-up yof the emission and plate current is then in the beginning possible only at or near the critical plate potential eek determined by the known magnetron formula 50 where ra is the radius of the anode plates and eek is the potential between the anode and cathode. If in starting from zero, the plate potential ea is gradually increased then the plate current i. begins at a certain voltage value 55 ea', and attains its maximum at the critical condition esk corresponding with the formula and with which also the optimum of the generation of oscillation coincides, and at a further increase of ea the plate current decreases again down to zero at ea". In contrast to the known glow cathode magnetron arrangements, the currentvoltage line of the secondary emission magnetron has a pattern similar to a resonance curve as emphasized by the shaded surface in Figure 1.

The action of a protective resistor having a value W and starting from .a fixed Voltage point namely from the feed voltage Es can be shown in the diagram in the form of a straight resistance line. This resistance line must originate in the voltage point En on the abscissa, and is to indicate the value of the plate potential in relation to is and reduced by the voltage drop iaW. The resistance line is determined by the equation:

and its inclination towards the abscissa is dia/deazl/W. In Figure 1 two resistance lines I and 2 are shown for different resistance values W but starting from the same voltage point Ea. If Ia variable series resistor is chosen, the resistance line can be turned about the turning point En. But if at constant W the fed voltage Ea is changed the resistance line will thereby be displaced parallel to itself.

In accordance with the magnetic eld Hk and the associated resonance wave to, the plate current resonance curve exists at a certain plate potential ene. The working points may then be placed at the two intersection points A1 and A1 of the straight resistance line i with the current resonance curve. When the high frequency oscillation performance is influenced in .any manner, for instance by withdrawing of energy, the current resonance curve which is very closely related to the ultra high frequency building up performances becomes attened out, and the two working points A1 and A1 move towards each other on the resistance line until nally only a single working point is left over. The same conditions prevail when at a xed current resonance curve, the resistance of the line 2 is correspondingly so chosen that the resonance curve just forms a tangent in the point A2. At this working point at the slightest flattening out of the resonance curve the oscillations break off immediately, because the current maximum falls below the straight resistance line, and the building up can no longer Vset in by itself. Therefore this adjusting which is extremely critical is entirely useless for practical purposes.

The conditions become still worse when disregarding the limitation as regards the yield of the cathode coatings having secondary emission on which Figure 1 is based, and which would in practice be desirable in connection with the above Viewpoints. In this case, the current resonance curve assumes the form shown in Figure 2, i. e. at a certain voltage value ea', the oscillations and therewith also the plate current increase in a jump and build themselves up immediately to very high values reaching far beyond the load capacity of the tube, so that therefore they can no longer be measured. The same also holds true as regards the potential Value ea". The resistance line now passes through the shaded current region between es Iand es", and cuts off the area lying above. Thus there remains as the only possible part of the resistance line useful for the adjustment of the operation, the part between the limit points A and A, on which the actual working point may move back and forth in accordance with the conditions of building up and the load conditions. It can thus be seen that the insertion of a series resistor as protection means actually limits the load of the tube and protects the tube against the danger of destruction, but the instabilities therewith encountered represent a serious disadvantage in the practical operation since the movement of the working point always entails greater or lesser variations in the frequency and intensity.

In order to eliminate these instabilities, a third parameter is introduced into the arrangement and plays a decisive role in determining the operating conditions. There serves as a parameter in accordance with the invention, an auxiliary magnetic field permeating the tube and which is coupled with the building up and emission performances. This eld is produced entirely or in part by a eld coil through which the plate current of the secondary emission tube passes. An arrangement set up on the basis of the principle of the invention is represented in Figure 3. I-Iere the magnetron tube R is provided with the cold flat cathode K having both sides covered with substance emitting secondary electrons, the said sides being positioned in the center of an anode cylinder divided into the two segments Si and S2. Both segments are connected to the lecher line L whose resonance potentials control the electron current. The anode circuit contains the magnetizing coil Sp concentrically surrounding the tube, and the protective resistor W. Both are bridged by condensers C1 and Ce in order to avoid low frequency disturbance oscillations.

In order better to Visualize the functioning and action of the new stabilization method, the diagram according to Figure 2 is completed by the introduction of the magnetizing characteristic which like the straight resistance line, represents the magnetic eld intensity I-I produced by the coil Sp and its relationship to the plate current in. ield must be proportional to the m-agnetizing current in, i. e. H :Kia whereby the proportionality factor K contains the magnetic data of the coil Sp such as the number of turns, and the cross section, as well as eventually also the magnetic resistance of an existing iron core. The magnetizing line resulting from any factor K is shown in Figure 2 extending towards the left from the zero point of the coordinates. The

magnetic eld produced by the currents is in the ,-7

ordinate can be read at the abscissa.

Now in order to couple this straight magnetizing line with the straight resistance line, and to project it into the current voltage diagram, above the abscissa scale for es the associated scale of the magnetic ield for H is shown. As stated above, both values are connected with each other through the critical magnetron formula .and therefore a quadratic division will be obtained for H when the voltage scale is divided in a linear fashion. Above this quadratic scale for H, the magnetizing characteristic forms no longer a straight line but a parabola, such as shown in dotted lines in Figure 2. The parabola intersects the straight resistance line in the point A0 and this is the only possible point to which the generator can adjust itself in the operating state, when the abovecritical magnetron It is obvious that the magnetic formula holds true. Otherwise a different law is to be relied upon when transforming the scale for the magnetic iield. The ultra high frequency potentials produced at the anode segments may cause considerable deviations compared with the magnetron laws for the static case, which for the sake of simplification have herein not been considered since they have principally no bearing on the principle of the invention.

Obviously the operating conditions must be so chosen that the intersection point Aois in the center' between es and ea, since it is in this place that optimum .conditions can be expected. To this end, on the one hand, the ultra high frequency tuning is available, and on the other hand, if a certain wave is to be produced, the adjusting can be carried out by means of E, E. or I-I. When changing W the resistance line turns about the Eri-point, While it is placed parallel to itself when Ea is changed. An analogous parallel displacement can also be obtained with the magnetizing parabola, in that an additional component I-Io is superimposed on the magnetic field produced by ia, said component may be obtained for instance by an auxiliary Winding Sp in Figure 3 fed with direct current, or by a permanent magnet. The additional eld causes a parallel displacement of the magnetizing parabola in the iii-axis such as shown for instance by the parabola drawn in dotted lines in Figure 2. In the case represented, the straight resistance line is intersected in the point A0 outside the range of oscillation in which no building up is possible. In order to turn the parabola about the Zero point of the coordinates, simply the proportionality factor K is to be changed such as can be done for instance by meansy of a variable parallel resistor P.

The example of the circuit herein given and the operating conditions herein discussed in connection with said circuit may suiiice for the elucidation of the principle of the invention, which in practice affords numerous variations and modifications. It should furthermore be mentioned that by introducing suiiiciently large time constants in the current control and voltage control,

for instance by using suiciently large condensers C1 and Ce, such. a high inertia can be imparted to the stabilization that it can no longer compensate for rapid fluctuations. In this connection a practical requirement is seen for instance if the transmitter by superimposing speech potentials on the Working potential ea for instance, is to be modulated eventually by means of the transformer T in Figure 3. If the modulation periods are essentially greater than the time constants of the control performance, the transmitter can be fully controlled if adjusted to a suitable Working point.

The operation of the tube is stabilized by bringing about an equilibrium between the secondary electrons produced at the cathode and the electrons received by the anodes. In order to bring about this equilibrium the resistance W and the magnetic coil Sp are relied upon. If for any reason, the discharge current increases, the eifective accelerating voltage ea at the anode segments will decrease as a result of the voltage drop in the resistance W. At the same time, there is an increase in the strength of the magnetic field` winding. The effect of the strengthened magnetic field is to decrease the orbits of the electrons so that fewer electrons are taken from the discharge and sent to the anode. At the same time, the decreased anode voltage causes fewer electrons to be received by the anode segments. These actions have a compensating eilect on the current through the utput circuit connected tothe anode segments. too great reduction in the discharge current due to the decreased anode voltage is compensated for by an increase in the number of secondary electrons produced by the electrons confined to the discharge space, and thus more electrons become available to increase the discharge current. All these factors help in stabilizing theanode currents when balanced against each other;

What I claim as new isl. An electron discharge device having an envelope containing a straight cathode coated with material for emitting electrons, and an anode surrounding said cathode for receiving electrons from said cathode, means for producing` a magnetic field parallel to said cathode and anode and of suflicient strength to cause electrons to impinge ing the strength of the magnetic field and the rate of secondary emission.

2. An electron discharge device comprising an envelope containing a straight cathode coated with material for emitting electrons and a pair of coaxial anode segments surrounding said cathode for receiving electrons from said cathode, a coil surrounding said electron discharge device for producing a magnetic iield parallel to said cathode and anode segments and of sufficient strength to cause electrons to impinge on the cathode to release secondary electrons, a load circuit connected to the anode of said electron discharge device and including a resistor and a second electromagnetic coil surrounding said electron discharge device for varying the intensity of the magnetic field and the rate of secondary emission.

3. An electron discharge device comprising an envelope containing a straight cold cathode coated with electron emitting material and a pair of coaxial segments surroundingk said cathode for receiving electrons from said cathode, a coil surrounding said electron discharge device for producing a magnetic eld parallel to said cathode and anode segments and of suiicient strength to cause electrons to impinge on the cathode to release secondary electrons, a load circuit connected to the anode of said electron discharge device and including a resistor and a second electromagnetic coil surrounding said electron discharge device for varying the strength of the magnetic field and the rate of secondary emission.

- HANS ERICH HOLLMANN.

However, any strong tendency for a 

